The HTLV-1 infects dendritic cells, monocytes and CD4+ helper T-cells, and oncogenically transforms CD4+ T-cells and causes adult T-cell leukemia/lymphoma (ATLL)—an aggressive and often-fatal hematological malignancy that is poorly responsive to most anticancer treatments, in 3–5% of infected individuals. The gene discussed is CD4; the disease is adult T-cell leukemia/lymphoma.